1. Field of the Invention
The present invention relates generally to a lithographic projection apparatus and more particularly to a lithographic projection apparatus including received dose control.
2. Background of the Related Art
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to structure that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. The required matrix addressing can be performed using suitable electronic structure. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
During exposure, the radiation system supplies a projection beam of radiation to irradiate a portion of the mask (or other patterning structure) and the projection system images the irradiated portion of the mask onto a target portion of the substrate. It is important that the total plane of the mask is irradiated with an equal dose. Variations in the intensity of the projection beam over the plane of the mask will cause a variation in the dose and hence a variation in the quality of the imaged target portions on the substrate. Variations in the intensity cause the critical dimension (i.e. the line width of the imaged lines) to vary, which is not wanted. Substrates having such a variation of the critical dimension may be rejected during a quality control in the manufacturing process. If the intensity of irradiation is higher than a nominal intensity at a certain portion of the mask, the critical dimension of the images projected from that portion will be smaller than when the nominal intensity is used at that portion.
One aspect of the invention provides an apparatus with a very low intensity variation over the plane of the mask (or other patterning structure). Accordingly, the present invention provides an apparatus including sensor constructed and arranged to measure a movement of the projection beam substantially perpendicular to its propagation direction and a dose controller constructed and arranged to control a received dose of said projection beam on a target portion of the substrate in response to an output from said sensor. The received dose is defined as the integral of the received intensities on a target portion of the substrate.
It has been determined by the inventors that one cause of variations of the intensity of irradiation of the mask is a movement of the projection beam perpendicular to its propagation direction. This movement may be caused by the radiation system mechanically moving with respect to the projection system or the source of the projection beam moving with respect to the projection system. The latter can, for example, be the case if a plasma source is used. The plasma that radiates the projection beam can be moving perpendicular to its propagation direction. The movements of the plasma will be projected through the radiation system of the lithographic apparatus and will cause the projection beam to move with respect to the projection system.
The invention can be advantageously used in a lithographic projection apparatus wherein said support structure is movable in a scanning direction that is substantially perpendicular to the propagation direction, and said sensor is adapted to measure movements of said projection beam in a direction corresponding to said scanning direction.
A lithographic projection apparatus as described in the previous paragraph can be very sensitive to movements in said scanning direction because the relative movement of the projection beam with respect to the mask (or other patterning structure) is determined by the movement of the mask plus the movement of the projection beam. The relative movement of the projection beam with respect to the mask will determine the intensity of irradiation on a portion of the mask. If for example, the mask moves in the same direction as the projection beam the relative movement of that beam with respect to the mask will be low and consequently the intensity of the irradiation higher on that portion of the mask. If, the other way around, the projection beam moves in the opposite direction to the mask the relative movement will be high and the intensity of the irradiation lower on that portion of the mask. The control structure may be adapted to control the movements of said support structure and said substrate table in the scanning direction, because by adjusting the speed of the mask and the substrate table the received dose of said projection beam on a target portion of said substrate can be controlled.
Control structure can be connected to the said sensor and to adjustment structure for adjusting the intensity of the projection beam in response to an adjustment signal from said control structure. The control structure can calculate an adjusted radiation intensity in response to information of the sensor about the movement of the projection beam with respect to the projection system. This adjusted radiation intensity (e.g. adjustment signal) will be sent to a source of the projection beam, which subsequently adjusts the intensity of that beam. If the projection beam is pulsed (e.g. the source radiates in pulses) said control structure can adjust the repetition rate or frequency of the pulses to adjust the intensity of that beam. Alternatively, said control structure may be constructed and arranged to adjust the energy per pulse.
If mechanical movements between the radiation system and the projection system cause the movements of the projection beam, the sensor may be adapted to measure mechanical movements. It may be advantageous to use acceleration sensor for such measurements. A direct mechanical connection between the radiation system and the projection system for the sensor may be avoided in this way.
If a movement of the source of the projection beam causes the movements of the projection beam the sensor may comprise intensity sensor for measuring the intensity of that beam. This can be done by using two or more light intensity measuring sensors, which are located at a fixed place in the projection beam and are used for a differential measurement. If the light intensity at a first sensor is increasing relative to the light intensity received by a second sensor the projection beam will be moving in the direction of the first sensor.
The sensor can be connected to a source sensor and said source sensor can measure movements of the source of said projection beam with respect to a reference point. The source sensor can be placed nearby the source (e.g. a plasma) and the results of the source sensor can be processed in the control structure such that the movements of the projection beam with respect to the projection system can be calculated.
In the case of a pulsed source, the sensor will give a measurement signal when the source of the projection beam emits a pulse. The position of the projection beam during each pulse will be measured and a future position of the projection beam with respect to the projection system can be determined by the control structure, if said movement is an oscillating movement. For this purpose the control structure may be provided with calculating structure for calculating a future position of said projection beam. When the oscillating movement is known and the calculating structure can calculate a future position the control structure can predict the position of the projection beam for a subsequent pulse. The said subsequent pulse will be triggered at a moment when the projection beam is at a required position with respect to the projection system. For the first pulses irradiated by the source the oscillation movement will not be known and therefore these pulses will be positioned randomly. To alleviate a possible non-uniformity in the intensity caused by the first pulses, a data storage device connected to said sensor and said control structure can be used. The data storage device will store information about the intensity and position of the projection beam during the randomly radiated first pulses. When the oscillating movement is known, for example after the first five pulses are radiated, this data can be used to trigger the next pulses for the projection beam such that any intensity variation during the first pulses is compensated during the following pulses.
According to a further aspect of the invention there is provided a device manufacturing method including providing a substrate that is at least partially covered by a layer of radiation-sensitive material, providing a projection beam of radiation, having a propagation direction, using a radiation system, using patterning structure to endow the projection beam with a pattern in its cross-section, projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material, wherein movements of the projection beam substantially perpendicular to its propagation direction are measured by sensor which are connected to control structure for adjusting the intensity of said projection beam in response to an output from said sensor.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.